Electronic phase-locked loops (PLL) have a wide range of applications in the field of electronics. An introduction to these techniques is presented in F. M. Gardner, Phaselock Techniques, 3rd ed. (Wiley, 2005). Phase-locked loops can be extended to the optical domain by use of semiconductor lasers as current-controlled oscillators, thereby realizing an opto-electronic implementation of phase-locked loops, as described by A. Yariv, in “Dynamic analysis of the semiconductor laser as a current-controlled oscillator in the optical phased-lock loop: applications,” Optics Letters, vol. 30, pp. 2191-2193, September 2005. The opto-electronic implementation of phase-locked loops is commonly referred to as optical phase-lock loops (OPLLs).
Extremely wide-band optical waveforms and precisely tunable Terahertz signals can be generated over a wide frequency range by using OPLLs to electronically control the frequency and phase of semiconductor lasers (SCLs) including near-visible and near-infrared semiconductor diode lasers and mid-infrared quantum cascade lasers (QCLs). Such electronic control enables a number of applications including coherent power combining (see, for example, N. Satyan, W. Liang, F. Aflatouni, A. Yariv, A. Kewitsch, G. Rakuljic, and H. Hashemi, “Phase-controlled apertures using heterodyne optical phase-locked loops,” IEEE Photonics Technology Letters, vol. 20, pp. 897-899, May-June 2008) and U.S. patent application Ser. No. 2006/0239312 to Kewitsch et al. Moreover, techniques to stabilize the frequency of semiconductor lasers are disclosed in U.S. Pat. No. 5,717,708 to Mells.
Semiconductor laser-based OPLLs are promising candidates for a number of applications in the fields of frequency modulated continuous wave (FMCW) laser radar, arbitrary broadband waveform generation, Terahertz signal generation, and coherent optical communications. For example, FMCW laser radar techniques are described in M.-C. Amann, T. Bosch, M. Lescure, R. Myllyla and M. Rioux, “Laser ranging: a critical review of usual techniques for distance measurement,” Opt. Eng. 40, 10-19 (2001), J. Zheng, “Analysis of Optical Frequency-Modulated Continuous-Wave Interference,” Appl. Opt. 43, 4189-4198 (2004), and W. S. Burdic, Radar signal analysis (Prentice-Hall, 1968), Chap. 5.
The wide gain bandwidth of the semiconductor quantum well media, the narrow linewidth of a single mode semiconductor laser (SCL), and the ability to electronically control the lasing frequency using the injection current make the SCL ideal for applications to a wideband swept-frequency source in a FMCW imaging system. However, the bandwidth and the speed of demonstrated frequency sweeps provided by prior art SCL's have been limited by the inherent non-linearity of the frequency modulation response of the SCL as a function of the injection current, especially at high tuning rates. In general, the rate of the frequency sweep was limited to about 100 GHz in 10 ms in the prior art.
In the prior art, SCL designs have been proposed to provide rapid, wide bandwidth tuning. See, for example, G. Beheim and K. Fritsch, “Remote displacement measurements using a laser diode,” Electron. Lett. 21, 93-94 (1983), E. C. Burrows and K.-Y. Liou, “High-resolution laser LIDAR utilizing two-section distributed feedback semiconductor laser as a coherent source,” Electron. Lett. 26, 577-579 (1990), A. Dieckmann, “FMCW-LIDAR with tunable twin-guide laser diode,” Electron. Lett. 30, 308-309 (1994), E. M. Strzelecki, D. A. Cohen, and L. Coldren, “Investigation of tunable single frequency diode lasers for sensor applications,” J. Lightwave Technol. 6, 1610-1680 (1988) and K. Iiyama, L-T. Wang, and K. Hayashi, “Linearizing optical frequency-sweep of a laser diode for FMCW reflectometry,” J. Lightwave Technol. 14, 173-178 (1996).
The prior art designs takes advantage of the dependence of the laser's emission frequency on drive current. The strong inherent tuning characteristic of the SCL, while offering the potential for wide tuning bandwidth, is a challenge to precisely and accurately control because of the resulting sensitivity of laser emission to both electronic noise and spurious optical power. Moreover, the semiconductor laser's gain dynamics result in relatively poor linearity, which negatively impacts its ability to generate precisely controlled optical waveforms. There is a significant need to develop semiconductor laser-based apparatus and methods that enable precise, repeatable, fast and accurate tunable optical waveforms.